Baluns are used to interface balanced systems to unbalanced systems, and to transition electrical energy therebetween. In fact, the word “balun” is derived from the “bal” of balanced and the “un” of unbalanced. Many antennas interface with an unbalanced feedline, such as a coaxial cable. A variety of antenna systems, such as a dipole antenna, are commonly regarded as balanced systems. However, in practice, such systems may exhibit some degree of voltage imbalance on the terminals of the antenna. As such, these antennas may be referred to as imperfectly balanced. As a result, when using a dipole antenna or other balanced antenna system, baluns are often added to transition balanced or imperfectly balanced terminal voltages of an antenna to unbalanced voltages of a feedline while maintaining equal and opposite currents at any instant of time in and out of the interface. The balun also transitions a balanced signal voltage transmitted or received by the antenna from or to the unbalanced voltage of a coaxial feed line.
Although some baluns transform impedances when transitioning between balanced to unbalanced systems, the main function of a balun is to provide proper isolation of current paths and voltage differences between balanced and unbalanced voltage systems. As one example, the need for a balun, and the isolation of paths provided by the balun, is seen when the balanced voltages of dipole antenna feedpoints are attached to unbalanced voltages of a coaxial feed line. While this example is of a dipole antenna, balanced and unbalanced also applies to other antenna systems and feedlines, which always must be someplace between being perfectly balanced and perfectly unbalanced in voltages while generally requiring exactly equal and opposing currents for optimum performance or satisfactory operation. In this example, a first dipole arm and a second dipole arm form a balanced or nearly balanced voltage load for the transmission line. The first dipole arm or balanced load terminal is attached directly to the inner conductor of the coaxial cable and the second dipole arm is attached directly to the outer conductor of the coaxial cable.
When any perfectly balanced voltage or imperfectly balanced voltage antenna system is operating without a balun and is connected to an unbalanced voltage transmission line, a first current flows in one direction at one instant of time through the first dipole arm and the inner conductor. At the same instant of time, a second current flows oppositely along the inside wall of the coaxial outer conductor and a portion reaches and flows into the second dipole arm. However, a third unwanted current develops where the second dipole arm is attached to the outer conductor of the unbalanced feedline. In this dipole example, an electrical voltage appears at the attachment point for the second current, and this voltage causes a third current and unwanted voltage to be created along the outer surface (or shield) of the coaxial cable. That is, the desired transmission line power is divided into two power components. The first power or energy component flows to or from the desired place known as the antenna, and a second unwanted power component appears from an undesired third current and voltage along the outside of the shield. As a result, the desired power is effectively divided into an unwanted and harmful power caused by unwanted current and voltage in an undesired place.
The creation of the third unwanted current results in unwanted and undesired radiation or reception from the feed line, and undesired unequal currents in the dipole arms. Such radiation and unequal currents consume power from the energy transferred between the antenna and the receiver, generator, or transmitter system, and, therefore, decrease efficiency and performance of the entire system. However, the magnitude of the disturbance in voltages and undesired third current depends on the impedance of the outside surface of the coaxial cable and the voltage driving that unwanted current. For example, if the impedance of the surface of the coaxial cable, antenna, other transmission line, or load is very high, then the amount of electrical current generated at the above-described transition point is low, and, therefore, the amount of useful and wanted electrical power converted into an undesired and harmful power is low. Consequently, when the impedance on the outside surface of a coaxial cable is high, the power is not divided, and the third unwanted current is effectively eliminated.
Therefore, if the impedance of the outside surface of the coaxial cable can be increased, then the radiation from the feed line and the unequal currents and voltages in the dipole arms due to the third current can be eliminated as a problem. To that end, the purpose of the balun is to increase the impedance along the outside surface of the transmission line, restricting unwanted diversion of useful power to useless or harmful power at the transition point.
Of particular concern is that impedance transforming baluns typically utilize two or more transmission line transformers that have equivalent construction and equivalent electromagnetic handling characteristics. However, during operation, the first transformer is required to dissipate only a fraction of the power that the remaining transformers are subjected to. As such, constructing each of the transmission line transformers to be equivalent is unnecessary, wastes material, and unnecessarily adds to the overall cost of producing the balun.
Thus, there is a need for a balun that includes one transmission line transformer that has a greater impedance than a remaining number of transmission line transformers. Additionally, there is a need for a balun that includes one transmission line transformer that is able to dissipate more power and withstand more electromagnetic induced stress than a remaining number of transmission line transformers. There is still yet a need for a transmission line transformer that uses a reduced amount core material than a remaining number of transmission line transformers.